How Sensitive Is Global Warming to Carbon Dioxide?

Another global warming denial talking point has—as so many have before it—apparently bit the aerosol.

This time it has to do with climate sensitivity and a concept called “forcing.” These factors are critical to taking action about climate change, but also somewhat subtle, which means that it’s easy to obfuscate about them … as we’ll see in a moment.

The basic idea is that the temperature of the planet responds differently to different things in the atmosphere. Greenhouse gases warm the atmosphere up, because they let through sunlight, which warms the Earth’s surface, which in turn emits thermal infrared light. But those greenhouse gases block that thermal infrared, preventing it from leaking out into space. The atmosphere warms up, which warms the surface … and you get global warming.

Some other substances, like aerosols (particulates that are suspended in the air) can reflect sunlight, cooling the Earth. Land use, cloud cover, and many other agents play into warming and cooling as well. All these different factors are called forcings, and some are stronger than others. What actually happens to our planet’s temperature depends on the interplay of these factors.

We know—and yes, we know—that on the whole, all these factors add together to increase the warming of the planet. Earth is heating up.

But how fast? That’s the important question. If the Earth is relatively insensitive to, say, an increase in carbon dioxide, we have plenty of time before things get bad. But if it’s actually very sensitive to that, the temperature will increase faster, and the clock is ticking.

Climate scientists call this the climate sensitivity, and they measure it in terms of how much the Earth’s average temperature goes up if the amount of CO₂ in the air is doubled. We know how rapidly CO₂ is increasing, so this gives us a timescale for action.

Most studies show that the climate sensitivity gives us about 3° Celsius of heating for a doubling of CO₂. Using different methods, some show a lower sensitivity, closer to 1.5°. That’s a big difference! Which is it?

A new paper just published in Nature Climate Change^* shows that the lower sensitivity estimate is wrong. Studies that return that lower result don’t examine the different forcings correctly, which biases their sensitivity calculation downward. When the same methods are used correctly, the new study found the sensitivity falls right into line with the other estimates of 3°.

Andrew Dessler, a climatologist who has examined this issue, made a short video that explains this pretty well:

You can also find good explanations of this by Dana Nuccitelli at the Guardian and Gavin Schmidt (one of the paper authors) at Real Climate.

The reason this is important is obvious; we need to know how sensitive the climate is to the various factors controlling it. But it’s important to note that many climate change deniers like to point to the lower sensitivity estimate to say taking action on global warming is premature. As this new study shows, that’s wrong. We need to take action now. Heck, we need to take action 20 years ago! But now is what we’ve got, and we need to act.

So, of course, cue the usual gang of scum and villainy. Real Climate has a follow-up article talking about how some media have managed to screw up the reporting of this result. Instead of reporting that the climate is actually more sensitive to forcing than some “lukewarmers” think, they actually focus on the idea that some factors (like aerosols) cool the Earth, as if this is the news.

The Daily Mail, which is to accuracy what the wood chipper was to Steve Buscemi’s character in Fargo, went with this headline: “Forget global warming, pollution can actually COOL the planet: Aerosols from burning fossil fuels may protect some regions from the effects of climate change.”

Yeah, except that’s not at all what the study was about. And the “forget global warming” part of the headline is simply false, and shameful.

Again, we’ve known this about aerosols for a long time. Volcanic eruptions, for example, spew a lot of aerosols into the air, and big eruptions can measurably cool things off … for a while. The presence of CO₂ and other greenhouse gases, however, will not be denied. The rate at which the Earth is warming over time far outstrips what any volcano can do. In the end, the temperature goes up. And up and up and up.

Global warming is real, and burying your head in the sand—or publishing grossly error-laden and transparently misleading articles—won’t make it go away. This new study shows once again that we need our politicians to take this seriously. With an election coming up (you may have noticed) this has never been more true, or more important. 

* Correction, Jan. 13, 2016: I originally misstated that the study was published in Nature, rather than Nature Climate Change.